The invention relates to electronic, contactless switchgear, having an externally modulatable component, preferably an externally modulatable coil, a switch amplifier connected to the output side of the component or coil, and an electronic switch, for instance a transistor, a thyristor or a triac, that is controllable by the component or the coil.
It has been stated above that an externally modulatable component is part of the electronic switchgear in question. The component may be any one of quite various kinds of externally modulatable components. In particular, however, the invention relates to electronic switchgear of the above-described type, in which the externally modulatable component is embodied as a coil, and the coil is part of the circuit of an externally modulatable oscillator having an oscillator amplifier and a feedback.
Electronic switchgear of the latter type mentioned above is embodied in contactless fashion and is now being used more an more, instead of electrical, mechanically actuated switchgear operating with contact, in electronic measuring and open- and closed-loop control circuits; this kind of switchgear is known as proximity switches. With such proximity switches, an indication is provided as to whether a modulating element, to which the corresponding proximity switch is sensitive, has come sufficiently close to the proximity switch. In other words, if a modulating element to which the corresponding proximity switch is sensitive has approached sufficiently close to the oscillator, then the oscillator toggles the electronic switch. In the case of switchgear embodied as a closing element, the initially non-conducting electronic switch now becomes conductive, while in switchgear embodied as an opener the initially conductive electronic switch is now made to block. With switchgear of this type, an indication can also be provided as to whether a physical variable of a modulating medium to which the switchgear is sensitive has attained a corresponding value.
A substantial component of switchgear of the above-described type is the externally modulatable oscillator, which is typically embodied such that it can be inductively modulated; in this case it is an inductive proximity switch.
In conventional inductive proximity switches, the following equations apply for the oscillator, as long as a metal part has not yet attained a predetermined distance from the oscillator: EQU K.multidot.V=1
where K=feedback factor and V=amplification factor; that is, the oscillator vibrates. Once the corresponding metal part has attained the predetermined spacing, the increasing damping of the oscillator leads to a reduction of the amplifier factor V, so that K times V becomes less than 1; that is, the oscillator stops vibrating (or the amplitude of the oscillator vibration drops below a predetermined threshold value).
Conventional inductive proximity switches, which respond in such a manner that the electromagnetic alternating field in the metal part that is used as the modulating element induces damping eddy currents in the vibrating oscillator, are still in need of further improvement in the following aspects.
On the one hand, the damping that is present when a metal part has not approached must be as low as possible, so that upon the approach of the metal part toward the inductive proximity switch, damping caused by eddy currents can be evaluated. This means that the conventional inductive proximity switches are not readily as sensitive as would be desired. In particular, the conventional inductive proximity switches must not be surrounded on all sides by metal, and such inductive proximity switches cannot detect ferromagnetic modulating elements through damping paramagnetic material, for instance through aluminum housing walls. On the other hand, the conventional inductive proximity switches respond to modulating elements both of paramagnetic and of ferromagnetic material.